my imac was working fine from 2011 till now, but suddenly when i played a movie i found that the internal left speaker is not producing the voice, i thought it can be in movie then i played a music and same thing happen, to ensure if its a software issue i plugged the headphone on the back of iMac and both channels worked fine, but after unplugging same thing happens, i tried restarting it and i notice that when imac start up it give a chime sound, it was also coming from right speaker, not the left speaker, i have checked the settings in system preferences> sound and the speaker balanced slider was in center position, i dont know why it happens and how i can rectify it, please help me